1. Field of the Invention
The present invention relates to registration of plural image exposures on a photoreceptive belt and, more particularly, to transverse registration of images formed by exposure to transverse line scanning on a longitudinally moving belt subject to lateral deviation from linear travel.
2. Description of the Related Art
In digital methods for providing color images by xerography, for example, each image area on a photoreceptive belt must make at least four passes relative to a transverse line scan by a modulated laser beam or a linear array of light emitting diodes
and registered to within a 0.1 millimeter circle or within a tolerance of .+-.0.05 mm. The use of timing marks spaced longitudinally on the belt in correspondance with the image areas, coupled with available electronic sensors and associated circuitry, has enabled longitudinal registration of the image areas, or registration in the direction of belt travel, acceptably within this degree of precision. Registration of image elements or "pixels" in a transverse direction, or in a direction perpendicular to the direction of belt travel, has been more difficult to attain due to manufacturing tolerances in belt width, supporting roller geometry, uneven belt stretch or a combination of such tolerances.
In the past, the problems associated with transverse or cross registration of pixels in multiple exposures of images on photoreceptive belts have been addressed either by attempts to guide the belt in a manner to avoid or at least reduce lateral shifting, or by corrective steering of the belt in response to sensed lateral deviation from true linear travel. Physical guiding of the belt edges is limited in accuracy to the belt width variations or tolerances and is not acceptable for cross registration within the limits required for high quality xerographic color image reproduction, for example.
Belt steering techniques previously employed have most commonly relied on belt edge sensors to detect lateral shifting of the belt and to control a mechanical steering mechanism for returning it to the correct line of travel. Such belt steering systems are subject to inaccuracy resulting from belt width tolerances and on this basis alone, fall short of a full solution of the problem. The use of printed indicia in various continuous patterns along a longitudinal margin of the belt has also been proposed and, while avoiding the problems associated with belt width variations, are prone to diminished detector signal accuracy due to interference caused by dirt which may accumulate on the printed indicia, particularly in the environment of a xerographic apparatus. Another major drawback with steering mechanisms generally employed to control lateral shifting of belts is the need for a mechanical steering mechanism which requires a substantial measure of belt travel and time before the electrically sensed belt shifting can be accomplished.
These and other problems associated with accurate cross registration of images exposed on photoreceptive belts have demonstrated an acute need for improvement to accommodate the high degree of resolution attainable with state-of-the-art electronic scanning equipment.